Kraft cooking method using polysulfide cooking liquor

Abstract

The method is for the preparation of kraft pulp with increased pulping yield from lignin-containing cellulosic material using polysulfide cooking liquor. In order to increase carbohydrate stabilization and hence the yield from a kraft cooking process a first impregnation stage using polysulfide cooking liquor is implemented at high alkali and polysulfide concentration and at a low temperature. Knots are added to a high-pressure conduit extending between an impregnation vessel and a digester.

We claim: 
     
       1. A method for the preparation of kraft pulp with increased pulping yield from lignin-containing cellulosic material using polysulfide cooking liquor in a continuous cooking system, comprising:
 providing an impregnation vessel in operative engagement with a digester via a high-pressure conduit, the impregnation vessel having a first impregnation stage; 
 heating a lignin-containing cellulosic material to a temperature in a range of 50-100° C. followed by adding polysulfide cooking liquor to the first impregnation stage which in turn is followed by cooking stages in the digester; 
 adding knots to the high-pressure conduit extending between the impregnation vessel and the digester; and 
 conducting the first impregnation stage at high alkali concentration above 60 g/l (effective alkali (EA) as NaOH basis) when adding the polysulfide cooking liquor, wherein the polysulfide concentration is above 3 g/l, or above 0.09 mol/l, when adding the polysulfide cooking liquor, wherein the first impregnation stage has a liquor-to-wood ratio in a range of 2.0 to 3.2 in order to increase a relative carbohydrate stability, the liquor-to-wood ratio calculated as containing polysulfide cooking liquor and wood moisture, and that the temperature is between 80-120° C. during a retention time resulting in a H-factor in a range of 2-20 of the first impregnation stage. 
 
     
     
       2. The method according to  claim 1  wherein the method further comprises providing a heater, the heater heating polysulfide cooking liquor prior to the polysulfide cooking liquor entering the impregnation vessel. 
     
     
       3. The method according to  claim 2  wherein the method further comprises using a heat exchanger and exchanging heat between black liquor withdrawn from the digester with the polysulfide cooking liquor to heat the polysulfide cooking liquor. 
     
     
       4. The method according to  claim 1  wherein more than 90% of the total charge of cooking liquor needed for completion of the cooking stages to the intended kappa number below 40 is charged to the first impregnation stage, and that at least 175 kg of effective alkali (EA as NaOH) for softwood and 160 kg of effective alkali for hardwood per ton of chips is charged. 
     
     
       5. The method according to  claim 4  wherein the alkali concentration is reduced by at least 8 g/l (EA as NaOH basis) by adding additional cooking liquids having lower alkali concentration than the alkali concentration prevailing at end of the first impregnation stage when increasing the temperature to cooking temperature, said cooking liquids in at least part thereof includes black liquor. 
     
     
       6. The method according to  claim 5  wherein no black liquor is added to the first impregnation stage. 
     
     
       7. The method according to  claim 6  wherein the white liquor added to the first impregnation stage has an alkali concentration above 100 g/l (EA as NaOH basis) and a polysulfide concentration above 4 g/l. 
     
     
       8. The method according to  claim 1  wherein the cooking stages in the digester results in a kraft pulp with a kappa number below 40.